In order to increase efficiency and performance of gas turbines so as to provide increased power generation, lower emissions and improved specific fuel consumption, turbines are tasked to operate at higher temperatures and under harsher conditions. Such conditions become a challenge for cooling of certain materials.
As operating temperatures have increased, new methods of cooling alloys have been developed. For example, ceramic thermal barrier coatings (TBCs) are applied to the surfaces of components in the stream of the hot effluent gases of combustion to reduce the heat transfer rate and to provide thermal protection to the underlying metal and allow the components to withstand higher temperatures. Also, cooling holes are used to provide film cooling to improve thermal capability or protection. Concurrently, ceramic matrix composites (CMCs) have been developed as substitutes for some alloys. The CMCs provide more desirable temperature and density properties in comparison to some metals; however, they present additional challenges.
Processing laminated composite turbine airfoils, such as with CMCs, has been shown to be effectively executed using melt infiltration (MI). However, often times, processing problems arise when the laminates become thick and/or the geometry becomes complex. Incomplete densification of the interior regions of the laminate will result when the matrix-forming infiltrant material (usually Si or an Si-based alloy) is unable to fill the entire preform during melt infiltration. The undesirable microstructure that forms is characterized by large lacks of infiltration, porosity, and voids. Typically, such defect structures result in large degradation of most mechanical properties, including interlaminar behavior, matrix-dominated properties such as proportional limit, and fiber-dominated properties such as tensile strength and ductility. All of which are undesirable.
These densification problems are especially acute when processing thick sections having certain microstructural traits such as very high fiber volume fractions, highly non-uniform fiber distributions, and tightly consolidated preform matrices. In some instances, these microstructural features tend to further slow infiltration rates during processing, exacerbating the densification problems and attendant mechanical property reductions described earlier. Additionally, densification of thick section components might benefit from longer process times at melt infiltration temperatures; however, such long processing times can damage the fiber-matrix interface and lead to unacceptable degradation of mechanical properties.